Parasympathetic nervous system
Webster, Douglas B. Department of Otorhinolaryngology and Biocommunication, Louisiana State University Medical Center, Baton Rouge, Louisiana.
Last reviewed:March 2021
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A portion of the autonomic nervous system. The autonomic nervous system, which functions primarily at a subconscious level, controls visceral functions of the body, including those associated with cardiovascular activity, digestion, metabolism, and thermoregulation. Traditionally, the autonomic nervous system is divided into the parasympathetic nervous system and sympathetic nervous system, based on the region of the brain or spinal cord in which the autonomic nerves have their origin. Predominantly, the parasympathetic system seems in control during pleasant periods, such as during digestion and rest. In contrast, the sympathetic system, by altering the level at which various organs function, enables the body to rise to emergency demands encountered in flight, combat, pursuit, and pain. The nerves of the parasympathetic system either exit the brainstem via cranial nerves or exit the sacral segments of the spinal cord (see illustration). Structurally, the parasympathetic nervous system consists of two neuron chains. The first neuron has a long axon and synapses with the second neuron near or in the organ innervated. In general, action of the parasympathetic nervous system is opposite to that of the sympathetic nervous system. The alkaloid pilocarpine excites parasympathetic activity, whereas atropine inhibits it. See also: Autonomic nervous system; Brain; Nerve; Neurobiology; Neuron; Spinal cord; Sympathetic nervous system
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